There is disclosed a power supply apparatus that includes a secondary battery used as a backup power source upon a power failure and supplies power from the secondary battery to load appliances in the event of a power failure (see, e.g., Japanese Patent No. 3322116).
A secondary battery needs to be periodically replaced because the capacity (Wh) thereof decreases with an increased use time. For example, since a secondary battery such as in a mobile phone is used while being repeatedly charged and discharged, the capacity of the secondary battery can be easily detected from the amount of discharged electricity and discharging time, or the amount of charged electricity and charging time. Therefore, the degree of the decrease in capacity relative to initial capacity is detected to determine the degraded state of the secondary battery, so that it is possible to detect the lifespan of the battery.
Meanwhile, since a secondary battery used as a backup power source upon a power failure is typically rarely charged or discharged except for a self-discharge portion, it is difficult to check the capacity of the battery. Further, since the secondary battery is used as a backup power source upon a power failure, it is impossible to check the capacity by discharging the secondary battery until the capacity thereof is decreased to about zero. Therefore, there is a need to check the capacity and to determine the degraded state of the secondary battery when the secondary battery is not in use while a power failure is not occurred.
As a test method of determining the degraded state of a secondary battery, there has been proposed a method for determining the degraded state of the secondary battery based on the voltage of the secondary battery obtained after a period of time has elapsed from the start of discharging as disclosed in Japanese Patent No. 3322116. However, this method determines degradation based on a variation in voltage before and after the discharging of the secondary battery and does not take into consideration charging/discharging time that varies with the degraded state of the battery.